Many books have been sanitised over the years, including Agatha Christie, Enid Blyton etc, so Twain is not the only one to lay in his grave gnashing his teeth...
Yes, I think it is sad that this has been done. Although the work is fiction, it represents a time and place and gives a backdrop to true history, as all historical novels written contemporaneously have done, although this was better written than most! Basically, this is revisionism and that just won't do.
I am loathe to sign up to a statement such as "political correctness gone too far!" though, as it is uncomfortably similar to headlines in the more seedy of UK tabloid newspapers. (Substitute "too far" with "mad" but the meaning remains similar.)
Douglas Bader was a war hero and has been lauded in books and films. Only the very early books and films noted that his dog was called Nigger though. We had neighbours when I was a lad who had a dog of that name. He kept buggering off and all the kids on the street used to wander down the road and nearby fields calling him.... me included.
Why do I mention this? Because innocence is just that. I had no idea of the derogatory use of the word till I went to the "big" school. If we hide behind offence, we learn to be offended.